mymoule_quicktabs_alter(&$quicktabs) { $quicktabs->default_tab=2; }

This works fine.

Please check if any JS script is overwriting your default tab. Normally if other module or JS script that alters the default tab and executes after current module; you might not see the effect in front-end.

mymoule_quicktabs_alter(&$quicktabs) {
  $quicktabs->default_tab=2;
}

This works fine.

Please check if any JS script is overwriting your default tab. Normally if other module or JS script that alters the default tab and executes after current module; you might not see the effect in front-end.